Craftsmen often need to mark or cut an elliptical shape into a workpiece. However, a true elliptical shape is difficult to inscribe and cut without the use of apparatus which assist in such inscription. Such apparatuses, generally designated "ellipse scribes," provide mechanical means whereby the craftsman may inscribe an elliptical shape into a workpiece.
Ellipse scribes are generally of two types. The first is an overhead ellipse scribe, illustrated in U.S. Pat. No. 3,808,691 to Chase. A swinging beam with a cursor at its end rests over the workpiece. The beam moves in an elliptical path over the workpiece and the cursor inscribes the ellipse thereupon. These ellipse scribes have the advantage that their components generally do not contact the workpiece, save for the cursor which performs the inscription, so the craftsman has an unobstructed view of the workpiece. However, overhead ellipse scribes also have the disadvantage that they must be of very great size if large ellipses are to be scribed. Thus, overhead ellipse scribes are impractical for the inscription of very large ellipses, such as those needed for elliptical tabletops.
The second common type of ellipse scribe is the contact or reciprocating beam ellipse scribe, as illustrated in U.S. Pat. No. 5,038,282 to Coll. These ellipse scribes utilize a beam having two pivotally mounted slides located thereupon, separated from each other by a distance. The slides reciprocate along mutually perpendicular tracks. As the slides reciprocate through a full cycle of motion, points along the beam follow an elliptical path, and thus a cursor mounted thereupon will scribe an ellipse. A cursor at one end of the beam (hereinafter the "scribe end") will inscribe an ellipse having a major axis radius equal to the distance between the cursor and the slide located the furthest away from the cursor, and a minor axis radius equal to the distance between the cursor and the slide located the nearest to the cursor. Coll's ellipse scribe includes a beam comprising two slidably attached ruled scales with a cursor slidably attached to the beam at the scribe end. Each scale within the beam includes a slide which is pivotally and slidably engaged to a respective track, with each track aligned perpendicularly to the other. As the slide of each scale traverses the entire track and returns to its starting point, the cursor sweeps out the ellipse. The scales may be repositioned with respect to one another within the beam so that the distances between their slides and the cursor may be reset, thereby altering the size of the ellipse to be scribed. The cursor may also be repositioned along the beam to alter the size of the ellipse.
While reciprocating beam ellipse scribes generally function in an acceptable manner, they have several disadvantages that the prior art fails to sufficiently address.
First, a reciprocating beam ellipse scribe must rest atop the workpiece upon which it will inscribe the ellipse. This can cause difficulties with the inscription of the ellipse because the tracks may intersect the elliptical path of the cursor, thereby requiring that the cursor be lifted over the tracks when they are encountered. Further, the center of the ellipse can be difficult to locate precisely when the ellipse scribe rests atop it.
Second, while the prior art discloses reciprocating beam ellipse scribes which include one or more scales for setting and measuring the axes of the ellipse to be inscribed, such scales tend to be difficult for novice craftsmen to understand and set correctly. This is because these scales generally include indicia indicating the length of either a major diameter, a minor diameter, a major radius, or a minor radius (or several of these). One who has never (or rarely) used an ellipse scribe before generally has difficulty determining whether he is setting the size of the diameter or the radius of the ellipse, and whether he is setting this parameter on a major or minor axis. Multiple scales help to compound the confusion. In addition, prior art ellipse scribes tend to have the scales integrally built into the apparatus, requiring the craftsman to use the mandated scale system (e.g. either a metric- or English-based scale which sets either the diameter or the radius of the ellipse) unless he wishes to endure the inconvenience of somehow modifying the ellipse scribe to use a customized scale.
Third, prior art ellipse scribes tend to be poorly designed for use with cutting equipment. Most tend to be designed for inscribing an ellipse on a workpiece using a cursor such as a pencil, rather than actually cutting the ellipse into the workpiece. These ellipse scribes adopt the notion that the craftsman can later use a power tool to cut out the inscribed ellipse. This frequently provides only a makeshift means for cutting a proper ellipse for a reason which unfortunately tends to become clear only after cutting of the inscribed ellipse on the workpiece has begun: it is generally extremely difficult to make high-horsepower cutting tools follow an inscribed elliptical path with high precision. Many cutting tools are intentionally or unintentionally designed to perform at their best when they cut along a straight path, making it difficult to force them to cut accurately along an elliptical path. In addition, with the size and configuration of many cutting tools, it can be difficult to even see such an inscribed path beneath the tool. Even where tools are designed so that a craftsman can see an inscribed line beneath the cutterhead, the cutterhead may be so large that precise alignment of the center of the cutterhead with the inscribed line is difficult. As an example, the wide cutterhead of a router can easily follow a thin inscribed line. However, the cut workpiece then tends to have a bumpy cut edge, since the cutterhead can waver laterally across the inscribed path while still remaining generally faithful to this path.
Fourth, the prior art ellipse scribes are too flimsy for practical use with power tools. As discussed above, they are primarily designed to support cursor means for writing ellipses on workpieces, rather than cutting an ellipse into a workpiece. Since such cursor means for writing are almost invariably lighter than cursor means for cutting, the ellipse scribes of the prior art are built with correspondingly lighter overall structure. As a result, these ellipse scribes do not have the structural integrity to support powerful cutting tools due to the weight of such tools. If such cutting tools are used with a prior art ellipse scribe, either the ellipse scribe collapses entirely, or the beam of the ellipse scribe tends to buckle, bend, and finally collapse.
This especially tends to be the case with prior art ellipse scribes that are sized to cut large ellipses, since their structures utilize long beams which have a greater tendency for buckling. Thus, the larger the ellipse to be cut, the greater the risk that the ellipse scribe will be destroyed during use.
Further, even while some prior art ellipse scribes can support extremely light cutting tools, e.g. portable, battery-powered hand tools, they still tend to lead to unsatisfactory results.
Initially, the use of light cutting tools, while possibly sufficient to cut ellipses from paper or thin wood or plastic, is totally inadequate for cutting ellipses from thicker or harder materials.
In addition, the structural integrity of the ellipse scribe still tends to be a problem. While the beam of the ellipse scribe may seem to adequately support a light cutting tool when the tool is at rest, problems arise when the tool is actually put to use. Cutting tools tend to generate tensile and compressive forces along the beam as the cutting forces generated by the interaction of the tool and the workpiece push and pull at the scribe end. This in turn leads to distortion in the beam and correspondingly distorted ellipses. This effect is especially apparent with cutting tools that cut at their best along a straight line. Since such cutting tools naturally resist following an elliptical path, they tend to cause the beam to bow upward and downward as they are moved along an elliptical path. This effect is also especially common in ellipse scribes with longer beams, i.e. those designed to cut larger ellipses.
The cutting forces generated by the cutting tool have an additional effect. The pushing and pulling of the cutting tool on the beam can generate shear forces of high magnitude at points along the beam. Most prior art ellipse scribes quickly or eventually break under these forces. Also, the shear forces can cause slippage in the mechanism that fixes the ellipse size. Thus, while the ellipse is being cut, the elliptical path can suddenly grow in size, and an elliptical spiral is cut instead. This slippage effect can be dangerous and also quite expensive, e.g. where the slippage causes a large workpiece to be cut improperly. This slippage effect is also common in prior art ellipse scribes because they have poor vibration damping properties, and therefore the vibrations from the cutting tool shake the size-fixing mechanism until it gradually slips.